In the duct there is only one way to expand the field; that is: at twice the distance the length of the covered arc is doubled. Since this may not be an ideal process, we can assume an attenuation factor to express it as A/r, but basically the nature of the beast is 1/r (i.e. -6 dB per doubled distance).
This condition also generates the peak activity on the known bands, as it is the time to look for ultimate DX. Therefore interference studies should be related to this factor, which makes that interference levels can be easily 40 dB or more higher than calculated with the